This invention relates to a vehicle window glass antenna, i.e. an antenna disposed on or in a vehicle window glass usually for receiving broadcast waves.
In recent automobiles there is a trend to adoption of a so-called window glass antenna, which means disposing conductive strips of an antenna pattern on a window glass, usually rear window glass, in addition to the conventional defogging heater strips.
Known automobile window glass antennas are classified into two types according to relation between the heater strips and the antenna strips. The first type of window glass antennas are characterized by electrical connection between the antenna strips and the heater strips t utilize the heater strips as auxiliary elements of the antenna. However, the antennas of this type have disadvantages and entail inconvenience in several aspects. There is the need of preventing the received wave from passing to the ground via the earth line for the heater strips and also preventing the DC current supplied to the heater strips from flowing to the feeding terminal of the radio receiver. Naturally the electric circuits relating to the window glass antenna become very complicated, and still there is some possibility of a short-circuiting accident attributed to the DC current applied to the heater strips when, for example, relatively thin coaxial cables are used. Besides, the radio receiver is liable to make a disturbing noise while the heater strips are energized.
In the second type of window glass antennas the conductive strips as the antenna elements are independent of the heater strips so that the disadvantages of the first type antennas are obviated. However, in this case it is a serious problem that average gain of the antenna in receiving either an AM broadcast wave or an FM broadcast wave is too low. Furthermore, in the case of receiving an FM broadcast wave the window glass antenna exhibits a highly directional pattern so that it becomes difficult to receive the FM wave from a desired station depending on the head direction of the vehicle. Also it is difficult to form an window glass antenna pattern which is fully effective for reception of FM broadcast waves in both the 76-90 MHz band used in Japan and the 88-108 MHz band used in many other countries. These problems of the second type window glass antennas are fundamentally attributed to the narrowness of the glass area left for the antenna pattern above an array of defogging heater strips.